1. Field of the Invention
This invention relates to projection lens systems for use in projection televisions and, in particular, to improved projection lens systems having reduced spherochromatic aberrations.
2. Description of the Prior Art
Projection lens systems for CRT projection televisions have undergone continuing development during the past fifteen years or so. As a result, many of today's CRT projection televisions are equipped with fast lens systems which provide wide semi-fields of view.
Projection lens systems of this type generally include three lens units, i.e., a "first" or "A" lens unit located on the image side of the lens system of weak power, a "second" or "B" lens unit following the first lens unit of strong positive power, and a include three lens units, i.e., a "first" or "A" lens "third" or "C" lens unit following the second lens unit of strong negative power. See, for example, Betensky, U.S. Pat. Nos. 4,300,817, 4,348,081, and 4,526,442.
To achieve half-angles of view greater than about 28.degree., a fourth lens unit (referred to herein as the "corrector" or "CR" (unit) is normally added between the strong positive and the strong negative units of the three unit configuration, i.e., between the B and C units. See Betensky, U.S. Pat. No. 4,697,892, and Moskovich, U.S. Pat. Nos. 4,682,862, 4,755,028, and 4,776,681. This additional unit usually does not have much optical power; however, it must have at least one aspherical surface to correct for aperture dependent off-axis aberrations like sagittal oblique spherical and coma.
Color images for projection televisions are normally obtained by combining images from three color CRTs, i.e., a red CRT, a green CRT, and a blue CRT. So that the images from the three CRTs will lie substantially on top of one another at the viewing screen, i.e., to minimize color fringing, projection lens systems used in projection televisions are normally corrected for transverse chromatic aberration, i.e., the variation with wavelength of the height of an image point above the optical axis. Transverse chromatic aberration is also known as the chromatic difference in magnification or simply lateral color. This correction is usually achieved by locating the lens system's stop in the proximity of the B unit, e.g., at the middle of the B unit.
For many applications, the projection lens system does not need to be corrected for longitudinal chromatic aberration, i.e., the variation with wavelength of the location along the optical axis of an axial image point. Longitudinal chromatic aberration is also known as axial chromatic aberration or simply axial color. When not corrected by the lens system, this aberration is dealt with by physically adjusting the location of the lens system and its associated CRT with respect to the screen, i.e., by adjusting the front and back conjugates to compensate for the change in focal length of the lens system with wavelength.
The phosphors used in commercially available CRTs do not emit light at a single wavelength. In particular, green phosphors have significant sidebands in blue and red. Similar polychromaticity exists for red and blue phosphors, but to a lesser extent.
For certain applications, such as, high definition television, data displays, or systems which operate at a high magnification, lens systems which are fully or partially corrected for axial color are needed to avoid visible color fringing and/or loss of image contrast as a result of the color spread of CRTs. See, for example, Betensky, U.S. Pat. No. 4,815,831, Kreitzer, U.S. Pat. No. 4,900,139, and Moskovich, U.S. Pat. No. 4,963,007. Such fully or partially color corrected lens systems, however, are more complex and thus more expensive than non-color corrected systems. Accordingly, these systems are often not used in consumer applications.
Wessling, U.S. Pat. No. 5,055,922, discloses a less expensive approach for addressing the color spread problem. In accordance with this approach a filter material that absorbs at least some of the undesired CRT sidebands is incorporated in one or more elements of the lens system. Although this approach significantly reduces the light intensity in the sidebands, it does not completely eliminate them. Also, the filter material approach does not change the lens system's overall aberration behavior.
In addition to lateral and axial color, lens systems can exhibit an additional wavelength dependent aberration known as spherochromatism. This residual-type aberration involves changes in a lens system's spherical aberration with changes in wavelength. Basic spherical aberration for an uncorrected positive lens causes rays further from the axis to focus closer to the lens than rays closer to the axis.
A well-corrected projection television lens system will generally have little spherical aberration for light having a wavelength of around 546 nanometers, i.e., light in the yellow/green range. However, because of spherochromatism, spherical aberration at other wavelengths is often significant. In particular, spherical aberration will typically be substantially undercorrected over most of the aperture for blue light and substantially overcorrected for red light. As a result, the image produced by the projection lens has less contrast than it would have if these spherochromatic aberrations were not present.